Question

Δ ABC is isosceles. Angles B and C are congruent. If m∠B = (5x + 5)° and m∠C = (7x - 25)°, Find m∠A. A) 10° B) 15° C) 20° D) 80°

Answer 1

7x-25=5x+5
2x-25=5
2x=30
x=15
75+5=80
180-160
20
C) 20

Answer 2

option C

`20\°`

Explanation:

we know that

An isosceles triangle has two equal angles and two equal sides. The two equal angles are the base angles and the third angle is the vertex angle

In this problem the base angles are angle B and angle C

so

m∠B=m∠C

substitute

`(5x+5)\°=(7x-25)\°`

`(7x-5x)\°=(5+25)\°`

`2x=30\°`

`x=15\°`

m∠B=
`(5x+5)\°=5*15+5=80\°`

Remember that the sum of the internal angles of the triangle is equal to
`180\°`

Find m∠A

m∠A=
`180\°-2*80\°=20\°`